Reconfigurable gas sensor architecture with a high sensitivity at low temperatures

ABSTRACT

A gas sensing device includes a dielectric substrate, a heater integrated into a first side of the substrate and an insulating dielectric formed over the heater. A gas sensing layer is formed on a second side of the substrate opposite the first side. Contacts are formed on the gas sensing substrate. A noble material is formed on a portion of the gas sensing layer between the contacts to act as an ionizing catalyst such that, upon heating to a temperature, adsorption of a specific gas changes electronic properties of the gas sensing layer to permit detection of the gas.

BACKGROUND

Technical Field

The present invention relates to gas sensors, and more particularly to aportable SnO₂/noble metal gas sensor architecture with an integratedheater for detecting hydrocarbons, such as, methane gas.

Description of the Related Art

Oxide based gas sensors are widely employed in many differentindustries, e.g., coal mines, fuel production, building intake airquality control, oil and gas (to measure and manage fugitive gases),etc. Commercial oxide based sensors have large power consumption due tothe elevated temperature required by the sensor element to achieve highsensitivity and selectivity. State of the art commercial gas sensorswork at temperatures between 400 degrees C. and 550 degrees C. The hightemperature requirement causes large power consumption that needs to besupplied to a heater element.

Selectivity of the sensor toward different gases is difficult toimplement, and factory based calibration may not hold in environmentswhere multiple gases are mixed in the air. On-board calibratingsolutions for the sensor can overcome erroneous reading or false alarmsin the field.

SUMMARY

A gas sensing device includes a dielectric substrate, a heaterintegrated into a first side of the substrate and an insulatingdielectric formed over the heater. A gas sensing layer is formed on asecond side of the substrate opposite the first side. Contacts areformed on the gas sensing substrate. A noble material is formed on aportion of the gas sensing layer between the contacts to act as anionizing catalyst such that, upon heating to a temperature, adsorptionof a specific gas changes electronic properties of the gas sensing layerto permit detection of the gas.

Another sensing system includes a substrate and at least one gas sensingdevice mounted on the substrate and including a heater integratedtherein, a gas sensing layer formed between contacts formed on the gassensing layer and a noble material formed on a portion of the gassensing layer between the contacts. A gas canister for storing a knowngas is coupled to the substrate, and an injector is configured torelease gas from the canister in accordance with a calibration signalsuch that the at least one gas sensing device is calibrated usinglocally released gas from the canister.

A method for fabricating a gas sensing device includes forming a heateron a first side of a dielectric layer; depositing an insulatingdielectric over the heater; forming a metal-oxide semiconductor gassensing layer a second side of the dielectric layer opposite the firstside; patterning contacts on the gas sensing substrate and depositing anoble material on a portion of the gas sensing layer between thecontacts to act as an ionizing catalyst such that, upon heating to atemperature, adsorption of a specific gas changes electronic propertiesof the gas sensing layer to permit detection of the gas.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a gas sensor with an integratedheater in accordance with the present principles;

FIG. 2 is a schematic diagram showing a heater with Yagi Uda antennaelements integrated therein in accordance with the present principles;

FIG. 3A is a diagram showing a heat distribution without Yagi Udaantenna elements;

FIG. 3B is a diagram showing a heat distribution with Yagi Uda antennaelements in accordance with one embodiment;

FIG. 4 is a cross-sectional view of a gas sensor having multiple noblematerial structures thereon for sensing different gases at differenttemperatures in accordance with the present principles;

FIG. 5 is a schematic diagram showing a system including a built-incalibration feature, and communication module to communicatemeasurements in accordance with the present principles; and

FIG. 6 is a block/flow diagram showing a fabrication method forfabricating a gas sensor in accordance with one illustrative embodiment.

DETAILED DESCRIPTION

In accordance with the present principles, devices for detecting ambientgases are provided. In one embodiment, to increase efficiency, thedevice provides a structure for reducing heater element temperature. Thestructure operates at a low power consumption range while theselectivity and sensitivity maintain operational values. In oneembodiment, the structure includes a metal-oxide-insulator structurewith a top active gas sensing element exposed to an environment or tothe atmosphere. Gas molecules absorbed on an active substrate dissociatecreating a change in conductance in an active channel that iselectrically measured.

Underneath the structure, a heater is integrated. The heater may includea patterned metal film that can heat up to about 700 degrees C., and thetemperature of the top active gas sensing element is adjusted toincrease sensor response specificity. Larger operating temperatures areassociated with higher power consumption, and for portable applicationsof the sensor powered by battery, the reduction in energy consumption ishighly desirable. The gas sensing structure can operate at a temperatureas low as between about 150 degrees C. and 250 degrees C. with a fastsensor response and a short recovery time when subjected to, e.g.,methane exposure. Conventionally, low operating temperatures areassociated with drifting of the sensor response, which made quantitativemeasurements difficult, where the signal is compared to a baseline.

In one embodiment, a sensor, in accordance with the present principles,includes a self-calibration feature where a known concentration of gas,e.g., CH₄, is released and the sensor reading is updated in a look-uptable. Periodic calibration of the sensor will ensure a reliablemeasurement over an extended period of time.

In a particularly useful embodiment, an SnO₂ layer is employed for thedetection of gas molecules in the air that can have a thin metal(preferably a noble metal, e.g., Pd, Co, Pt, Ag, etc.) coating on thetop, or the coating may include nanoparticles dispersed orself-assembled on the surface to act as a catalyst and increasespecificity. The metal catalyst makes the detection specificity of thesensor significant and permits the sensor to distinguish betweendifferent chemical species. Differential measurements of multiplesensors that have catalyst of different metals permit consecutivemeasurements and can distinguish the different gas species.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

A design for an integrated circuit chip may be created in a graphicalcomputer programming language, and stored in a computer storage medium(such as a disk, tape, physical hard drive, or virtual hard drive suchas in a storage access network). If the designer does not fabricatechips or the photolithographic masks used to fabricate chips, thedesigner may transmit the resulting design by physical means (e.g., byproviding a copy of the storage medium storing the design) orelectronically (e.g., through the Internet) to such entities, directlyor indirectly. The stored design is then converted into the appropriateformat (e.g., GDSII) for the fabrication of photolithographic masks,which typically include multiple copies of the chip design in questionthat are to be formed on a wafer. The photolithographic masks areutilized to define areas of the wafer (and/or the layers thereon) to beetched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, an illustrative sensordevice 10 is shown in cross-section in accordance with the presentprinciples. The device 10 is formed on an insulating substrate 16. Theinsulating substrate 16 provides thermal insulation to prevent heat lossfrom an integrated nanoheater or heating element 18. The insulatingsubstrate 16 may include a ceramic material, glass or other materialcapable of handling operational temperatures of the sensor device 10.

In one embodiment, a three layer architecture is employed. A dielectriclayer 12 is formed over the nanoheater 18 and the insulating substrate16. The dielectric layer 12 may include SiO₂ or other dielectricmaterial and prevent current leakage from the heater element to the gassensing layer. The dielectric layer 12 may include a thickness of about200 nm, although other thicknesses are also contemplated. A gas sensingsubstrate 14 is formed over the dielectric layer 12. The gas sensingsubstrate 14 may include a SnO₂ layer of about 100 nm in thickness,although other thicknesses may be employed. On the top of the gassensing substrate 14, a metal layer 24 is deposited that facilitates gasadsorption. The metal layer 24 preferably includes an inert or noblemetal, such as Au, Ag, Pd, Pt, Rh, Co, or the like. One particularlyuseful metal includes Pd, which is sensitive for methane detection. Themetal layer 24 may include a thickness of about 3 nm, although otherthicknesses may be employed. The metal layer 24 can be broken up into ananoparticle layer by thermally annealing the structure after beingformed.

Contacts 26 are formed and patterned on the gas sensing substrate 14.The contacts 26 may include a bilayer structure comprised of aninterface layer 20 and a contact layer 22. In one embodiment, theinterface layer 20 may include Ni and may be about 100 nm in thickness,although other materials and thicknesses may be employed. The contactlayer 22 may include Al and is about 100 nm in thickness, although othermaterials and thicknesses may be employed.

The nanoheater 18 can tune the temperature of the gas sensing substrate14 (e.g., SnO₂ layer). In many situations, when the nanoheater 18generates heat, the heat could be absorbed by the underlying insulatingsubstrate 16 making uniform heating of the top surface difficult. Ingeneral, the heat emission is highly localized above the heatersubstrate forcing the heater to be similar in size to the gas sensinglayer to ensure uniform temperature for the sensing element.

In accordance with one aspect of the present principles, the heatdistribution is improved by using an antenna structure (e.g., Yagi Udaantenna structures) to extend the area where the heat is spread. A YagiUda antenna may be coupled to the heater 18 to provide efficientcoupling of the infrared radiation (heat) to the gas sensing substrate14, minimizing the heat absorption by the insulating substrate 16. Thenanoheater 18 may be sized to reduce power consumption, e.g., widthshould be reduced while increasing its length, to focus heat energy. Toovercome inefficient heat transfer between the heating element 18 andthe gas sensing layer 14, the heater width is tuned to change thedirectional emission of the nanoheater 18 utilizing plasmonicresonances. For example, to avoid heat from being absorbed by theunderlying insulating substrate 16, the nanoheater 18 is structured todirect heat upwards using the antenna structures. Therefore, to reducepower consumption, the nanoheater 18 width can be reduced whileincreasing its length (into the page of FIG. 1) to provide a moreuniform heating distribution for the gas sensing layer 14. The antennaelements will amplify the heat distribution, and their placement andoperation will increase heat transfer efficiency.

Referring to FIG. 2, a top perspective view shows the nanoheater 18relative to Yagi-Uda antenna elements 38. The nanoheater 18 may beconfigured in a plurality of ways. In one embodiment, the nanoheater 18includes an electrical element 34 (e.g., a resistor) in which current ispassed to generate heat. The electrical element 34 connects toelectrical contacts 30 (coming out of the page) using conductors 32. Inaccordance with the present principles, a more uniform heat distributioncan be achieved using an infrared Yagi Uda based antenna 36 that tunesthe infrared radiation emission based on periodic spaced elements 38.The elements 38 may include Pt or other metals.

In FIG. 3A, an illustrative heat distribution pattern 40 is shown for ananoheater 18 employed without the Yagi Uda based antenna. In FIG. 3B,an illustrative heat distribution pattern 42 is shown for a nanoheater18 employed with the Yagi Uda based antenna elements 38. An upper heatemission pattern 44 is more evenly distributed over the device as shownin FIG. 3B.

Referring again to FIG. 2, the distance between heater 18 and Yagi Udaantenna elements 38 and also the size of the Yagi Uda elements 38 can beemployed to tune an emission wavelength of the heaters 18 and be tunedtoward a wavelength where methane gas absorbs light. For example, aheater size could be about 500 nm wide and have a length of about 2microns while the spacing between the antenna elements 38 could bebetween about 2 and 3 microns. The spacings can be tuned such thatwavelengths are sensitive to different gases and adjusted for differentgases, e.g., methane (CH₄), CO₂, CO, etc.

Referring again to FIG. 1, during operation, the sensor device 10 can beexposed to air. Different gases like methane can be adsorbed at thesurface of the gas sensing substrate 14 (e.g., SnO₂) and react withelectrons which produce ionized gas. The ionized gas (e.g., oxygen,etc.) is responsible for a band bending shift that creates a depletionlayer around the grain boundaries of the gas sensing substrate 14 (e.g.,SnO₂). Gas molecules to be sensed in the air will be adsorbed to thesurface and react with the ionized oxygen or other gas. This reactioncauses a change in the depletion layer which reduces the conductivity ofthe gas sensing substrate 14 (e.g., SnO₂).

In one embodiment, the sensor device 10 includes a metal-oxidesemiconductor gas sensor with the heater 18 modified to be a Yagi Udaantenna for the detection of methane gas. The Yagi Uda antenna elementsare positioned at a distance that enhances emission, e.g., around 3.4microns (the methane absorption wavelength). The antenna elements 38 areconfigured to have an aspect ratio larger than twenty and dimensionslarger than the emission wavelength. The sensor device 10 is operated atabout 150-250 degrees C. (using the heater 18) with a highdiscrimination of CH₄ against CO₂ and CO.

The gas sensing substrate 14 includes an ultrathin SnO₂ sensing layerwith a thin metal (e.g., Pd, Pt, Rh, etc.) layer 24 that may be brokenup into nano-particles through thermal annealing. The break-up of themetal film 24 into particles will increase surface area and theprobability of absorbing CH₄ molecules. A lower detection limit is alsoincreased in this way. The sensitivity is further enhanced bycontrolling the size of SnO₂ grains of the substrate 14 that aredeposited. In one particularly useful embodiment, particle size may beabout 100 Angstroms or less in diameter.

Referring to FIG. 4, another embodiment shows a sensor device 100 havinga metal film 110 forming a plurality of different catalyst materials(102-108) on the gas sensing substrate 14. The device 100 may includethin metals for catalyst materials 102-108, e.g., catalyst 102 mayinclude Pd, catalyst 104 may include Pt, catalyst 106 may include Rh,catalyst 108 may include Pt—Ag, etc. There may be one or more catalysts,and the catalysts may include pure metals, metal alloys, combinations ofmetals, etc. The sensitivity of the device 100 may be configured byusing different combinations and geometries of noble materials as thecatalysts on the sensor surface. The thickness and the surface to volumeratio of the noble materials of the film 110 is responsible for thesensitivity of the sensor 100. For example, using a thin layer of, e.g.,Pd (3 nm) as a surface catalyst the sensor 100 shows a high sensitivityspecifically to methane, etc.

The metal film 110 can be patterned such that the surface to volumeratio is increased. In one example, a nanoporous membrane is createdfrom the film 110 based on anodic reduction. Alternatively,nanoparticles can be deposited on the surface, or the film 110 can bepatterned using e-beam lithography. With different sizes and thicknessesof noble materials, the sensitivity, at low operational temperatures,can be adjusted for different gases.

To calibrate the system, a set of measurements are performed where thesensor response to different gas concentration is established:

$\begin{matrix}{{\frac{\Delta \; R}{R_{0}} = {\left( {CH}_{4} \right)^{a}\left( {CO}_{2} \right)^{b}{\exp \left( \frac{E_{a}}{k_{B}T} \right)}{\exp \left( {c \cdot {RH}} \right)}}},} & {{Eq}.\mspace{14mu} 1}\end{matrix}$

where ΔR is the change in resistance as methane gas is absorbed in thesurface; R₀ is reference resistance of the sensor with no methane gas inthe air, k_(B) is the Boltzmann constant, T is absolute temperature, RHis relative humidity, a is scaling factor, b is scaling parameter, c isa scaling parameter, E_(a) is the activation energy, (CH₄) and (CO₂) areconcentrations of the respective gases. The values for a, b, c, E_(a)are determined for all different materials by fitting change inresistance to changes caused by the well-known gas exposures. Eachcatalyst (e.g., Pd. Pt, Rh, etc.) and sensor geometry will have a set ofparameters (a, b, c, E_(a)). The sensor 100 will be characterized withrespect to principal components analysis where the discrimination ofdifferent sensor geometry and catalyst particles to different gases isquantified. A set of measurements may be based on parameters, and adetermination of a best fit of the sensor response to all possiblecombinations of parameters.

Device 100 may be employed to detect different gases under differentconditions. For example, a set of parameters may be adjusted to detect afirst gas, then another set of parameters are adjusted to detect asecond gas and so on. In one example, a gas may be sensitive to aparticular noble material and a temperature. Another gas may besensitive to the same noble material at a different temperature, and soon.

When multiple sensors are deployed at the same time where each sensorcan have a specific catalyst layer (102-108) and is initially calibratedor tuned to be sensitive to a specific gas either by heater design(pitch of antenna elements), catalyst particle types or operatingtemperatures. An atmospheric composition may be reconstructed from amulti-sensor measurement where each sensor will measure one or twocomponents of the environmental gases at the same time. In oneembodiment, multiple sensors (on a same device or on different deviceworking together) may have calibrated responses to different gases wheredifferent catalyst particles on the surface will have a specificresponse to different gases. The sensor may have a differentdiscriminated sensor response to each different gas. In one embodiment,a first sensor temperature may be employed to measure a first gasconcentration and then changed to a second temperature to measure for asecond gas. Each gas may react with a same catalyst or a differentcatalyst on the gas sensing layer.

In accordance with the present principles, gas sensing devices 10, 100,include open transistor architectures including a fine grain sensingsubstrate 14 (e.g., 100 Angstrom grain structure) for a SnO₂ film withnano-particles (thin metal film 24, 110 formed thereon. The device canbe operated at temperatures as low as 150 degrees C. with sub ppmsensitivity toward methane. The nano-particles of the metal film 24, 110can be tuned by material choice, geometry, etc. to be specific todetection of particular gases. In useful embodiments, multiple sensorsmay be operated simultaneously to distinguish between multiple gasesthat may be present (e.g., CH₄, H₂S, SO₂, etc.). The sensors 10, 100 mayinclude an integrated Yagi Uda antenna that is tuned to emit infraredradiation (e.g., at 3.4 microns for methane) by positioning the metalantenna elements around the nanoheater 18. The size of the sensor ispreferably narrower than the Yagi Uda antenna such that uniform heatingis provided. The sensor device or devices may be part of a largersystem. The system may include multiple sensors and may include a nearbyintegrated gas calibration module that permits periodic recalibration ofthe sensor or sensors. The sensor or sensors may be integrated with awireless sensor network where each sensing node may include amicroprocessor and a radio.

Referring to FIG. 5, a schematic block/flow diagram shows a sensorsystem 200 in accordance with one embodiment. The sensor system 200 maybe implemented on a substrate 212, such as a silicon substrate as anintegrated circuit or a printed wiring board. The sensor system 200includes a sensor device 10, e.g., a volatile organic compound (VOC)sensor. The sensor system 200 includes a self-calibration capabilitybuilt-in, where a calibration gas included in a canister or storagedevice 202 is employed periodically to evaluate a resistance change inthe gas sensing substrate (e.g., SnO₂ film) to one or more gases topermit calibration. The calibration gas is provided using an injector ormicropump 204, which may include a piezoelectric nanoliter injectorsimilar to an inkjet printhead.

To calibrate the sensor response the container 202 containing awell-known gas concentration is included on the same board 212 where asmall amount of gas is released to calibrate the sensor response. Themicropump 204 releases the gas, e.g., propane, methane, etc. The sensorreading is updated in memory 216 to provide a reliable reading. Bothtemperature and relative humidity may be measured by atemperature/humidity sensor(s) 218, which are recorded as a way toassess a shift in sensor readings in response to environmental changes.

The small canister 202 of a surrogate gas is released throughcontrolling a piezoelectric controlled nanoliter injector 204 to releasea controlled amount of gas to calibrate the VOC sensor 210. A 0.5 mlcanister will last the life-time of the device. More than one canistermay be employed. The multiple canisters may include a same or differentgases.

A controller or processor 206 (e.g., a microprocessor) may initiate acalibration method which intermittently activates the injector 204 torelease the calibration gas to calibrate the sensor 210. The sensor 210may be designed as an array of sensors where the catalyst can be Pd, Pt,Rh, etc., and each of the metals has an increased sensitivity toward aset of gases at a given temperature. The gases may be sensitive todifferent combinations of catalysts and temperatures. The array candiscriminate between different gases, e.g., CH₄, H₂S, CO₂, CO, etc.Sensors 210 can be connected in series or in parallel for single orpulsed operation.

Sensor 210 can be operated in a continuous powered mode or in atransient mode where a small heating pulse is applied, and the system200 will start to warm up but the measurement will be taken before thesensor 210 reaches its equilibrium temperature. Pulsed operation cansignificantly reduce the power consumption of the sensor element. Themicroprocessor 206 can provide a pulse sequence that provides fortransient heating of the sensor 210. When all sensors are operating inpulsed mode a same voltage pulse can power all sensors and measure theirtransient response to extract multi-component gases. The sensor 210 orsensors are operated in a pulsed mode to minimize power consumption.

Low power consumption opens the possibility to apply the sensor 210 inportable devices or as part of a wireless sensor network where thesensor will be triggered to acquire one measurement once an event issignaled. The system 200 may include a communications module 214 (e.g.,a radio) to communicate with the wireless sensor network or any suitablenetwork.

A distributed sensing system (not shown) using sensors 201 or system 200has applications in oil and gas industry, environmental tracking, andother gas monitoring applications, e.g., applications where multiplesensor readings are fused (over the Internet) with physical models toassess contamination levels, etc. In one example, a gas frackingoperation may need to monitor methane gas leaks to be documented on aperiodical basis. Similar applications are in livestock monitoring toquantify methane gas generation.

One particularly useful application includes employing the sensingsystem in the gas extraction industry to ensure compliance and open thepossibility to manage sensing data in a data store, provide analyticsusing analytics servers and cloud solutions to companies. This wouldassist in ensuring compliance with Environmental Protection Agency (EPA)regulations and ensure employee safety around extraction points.

One or more CH₄ sensors, in accordance with the present principles, maybe deployed nearby a gas extraction well where CH₄ escape is monitoredin real-time. Each sensor may be battery operated or otherwise poweredand be part of a multimodal distributed sensor network (e.g., a meshnetwork). The network may include other sensors and equipment, e.g.,optical sensors, anemometers, a remote gateway and cellular modems forcommunications, etc. The sensors may be connected to or be integratedwith a microprocessor and a radio, and sensor information may betransmitted wirelessly and collected across a large area using links toone or more different networks. Such networks may include a satellitecommunications, a cellular communications, wide area networkcommunications, local area networks communications, etc.

Network computers may be employed to store, process and distribute dataas needed. For example, data may be sent to clients, a data store,analytics computers, uploaded to links (for other networks), etc. Thedata may be combined with other information to provide additionalcontext. For example, weather data, e.g., from a satellite, may becombined or reported with the analytics or with the raw data readings toprovide comparison context or a better understanding of the readings.For example, an anemometer or wind sensor can indicate the direction offlow. A sensor in the path of wind flow will sample more often. Once athreshold is reached, the sensor could send a command to a centralcomputer that would wake up all other sensors. The central computer cancontinuously assign a sensor the task to sniff the air. The sensor maybe in the direction of the wind. If a threshold level is reached, thesensor will send a message to the central computer that will provide aschedule to localize leak and concentration level at the leak. Thesensor(s) provide low power operation and employ a low operatingtemperature. At a temperature of about 150 degrees C. a sensitivity ofless than 50% for 10 ppm methane can be achieved. The sensor can detectmethane in the ppb range.

Multiple measurements may be taken that are fed into a dispersion modelstored on a computer to localize leaks and assess the leak size and flowrate for the gas escape. Data may be combined with observation from asatellite to distinguish if the leak has no natural causes, like comingfrom a swamp or agricultural locations to eliminate false alarms. Analarm may be sent out if the leak level is above the regulation setlevel.

Referring to FIG. 6, in accordance with the present principles, gassensors are provided in transistor form. The transistors have adjustablesensitivity/selectivity and operate at low temperatures (e.g., 150degrees C.) using a thin film SnO₂ semiconductor gas sensor with lowpower consumption. Thin film architecture includes noble metal catalystmaterials and an integrated nanoheater for reduced power consumptionbased on tunable emission wavelength that would enhance the selectivityof gas absorption on the catalyst. An illustrative method is describedin forming the gas sensors in accordance with the present principles.

In some alternative implementations, the functions noted in the blocksmay occur out of the order noted in the figures. For example, two blocksshown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts or carry outcombinations of special purpose hardware and computer instructions.

In block 402, a dielectric wafer is provided or formed, which mayinclude SiO₂ layer on the top of a thickness of about 200 nm. In block404, a nanoheater is formed on the wafer. The nanoheater may be formedusing a metal, such as Pt, although other material may be employed. Thenanoheater may include a Yagi Uda antenna formed on the wafer. In block406, an insulating layer is deposited over the nanoheater. Theinsulating layer may include a 100 nm SiO₂ layer formed using e.g.,chemical vapor deposition (CVD).

In block 408, a gas sensing layer is formed. The gas sensing layer mayinclude a 100 nm SnO₂ layer formed on the wafer opposite the nanoheater.In an example of block 416, the gas sensing layer may be formed by adeposition process using an RF Magnetron Sputtering process set to apower of 125 W, a 99.99% SnO₂ target, a Ar process gas pressure of 6.5mTorr and a base chamber vacuum of 1×10⁻⁷ Torr. The chamber may bepumped using, e.g., a Turbo molecular pump. The time for a deposition ofthe 100 nm SnO₂ with at a rate of 0.57 \AA/sec at room temperature was1740 s. The sheet resistance of the 100 nm SnO₂ sample was about 2500Ohms/Square.

In block 410, contacts are formed. To improve adhesion and formcontacts, 100 nm Ni may be employed, followed by 100 nm Al as contactmetals. The contacts can be formed using a metal mask and E-beamevaporation. The contacts are improved if a 100 nm thickness of Ni andAl are employed on the SnO₂ layer. In block 412, noble materials(metals) are deposited and patterned on top of the gas sensing layer(SnO₂) with E-beam evaporation and a metal mask. In one example, thenoble materials include Pd, Pt, Rh, Ag, Au, combinations of metals(Pt—Ag), etc.

In one embodiment, a slug with 99.99% purity of Pd may be employed withthe E-beam evaporation process. A base chamber vacuum of 1×10⁻⁷ Torr wasemployed to minimize contamination from other metals and improveadhesion of metal layer. The deposition rate of 0.1 nm/s was performedto achieve a 3 nm metal layer of Pd and ensure continuous film growth.Other metals and geometries may also be provided.

In block 416, the noble metals can be processed to increase surface areaand improve gas sensitivity. In one embodiment, this is achieved bythermal annealing to break up the layer into nanoparticles.

Having described preferred embodiments for reconfigurable gas sensorarchitecture with a high sensitivity at low temperatures (which areintended to be illustrative and not limiting), it is noted thatmodifications and variations can be made by persons skilled in the artin light of the above teachings. It is therefore to be understood thatchanges may be made in the particular embodiments disclosed which arewithin the scope of the invention as outlined by the appended claims.Having thus described aspects of the invention, with the details andparticularity required by the patent laws, what is claimed and desiredprotected by Letters Patent is set forth in the appended claims.

1. A gas sensing device, comprising: a dielectric substrate; a heaterintegrated into a first side of the substrate; an insulating dielectricformed over the heater; a gas sensing layer formed on a second side ofthe substrate opposite the first side; contacts formed on the gassensing substrate; and a noble material formed on a portion of the gassensing layer between the contacts to act as an ionizing catalyst suchthat, upon heating to a temperature, adsorption of a specific gaschanges electronic properties of the gas sensing layer to permitdetection of the gas.
 2. The device as recited in claim 1, wherein theheater includes Yagi Uda antenna elements configured to distribute heatto the gas sensing layer in a uniform manner.
 3. The device as recitedin claim 2, wherein the Yagi Uda antenna elements are spaced apart adistance corresponding to an absorption wavelength of a gas to bedetected.
 4. The device as recited in claim 1, wherein the gas sensinglayer includes a metal-oxide semiconductor structure.
 5. The device asrecited in claim 4, wherein the metal-oxide semiconductor includes SnO₂as a gas sensing layer.
 6. The device as recited in claim 5, wherein thegas sensing layer is configured with grain boundaries sized to increasesurface area and sensitivity to a gas to be detected.
 7. The device asrecited in claim 6, wherein the gas to be detected includes methane andthe grain boundaries are about 100 Angstroms or less in diameter.
 8. Thedevice as recited in claim 1, wherein the specific gas includes methaneand the noble material includes Pd.
 9. The device as recited in claim 1,wherein the temperature is between about 150 to about 250 degrees C. 10.The device as recited in claim 1, wherein the noble material includesdifferent metals configured to be sensitive to detection of differentgases at one or more different temperatures.
 11. A sensing system,comprising: a substrate; at least one gas sensing device mounted on thesubstrate and including a heater integrated therein, a gas sensing layerformed between contacts formed on the gas sensing layer and a noblematerial formed on a portion of the gas sensing layer between thecontacts; a gas canister for storing a known gas coupled to thesubstrate; and an injector configured to release gas from the canisterin accordance with a calibration signal such that the at least one gassensing device is calibrated using locally released gas from thecanister.
 12. The system as recited in claim 11, wherein the heaterincludes Yagi Uda antenna elements configured to distribute heat to thegas sensing layer in a uniform manner.
 13. The system as recited inclaim 11, wherein the gas sensing layer includes a metal-oxidesemiconductor.
 14. The system as recited in claim 11, wherein the gassensing layer is configured with grain boundaries sized to increasesensitivity to a gas to be detected.
 15. The system as recited in claim11, further comprising a controller for generating the calibrationsignals and a communications module for transmission of gas measurementdata to a network.
 16. The system as recited in claim 11, wherein the atleast one gas sensing device operates at a temperature of between about150 to about 250 degrees C.
 17. The system as recited in claim 11,wherein the noble material includes different metals configured to besensitive to detection of different gases at one or more differenttemperatures.
 18. A method for fabricating a gas sensing device,comprising: forming a heater on a first side of a dielectric layer;depositing an insulating dielectric over the heater; forming ametal-oxide semiconductor gas sensing layer a second side of thedielectric layer opposite the first side; patterning contacts on the gassensing substrate; and depositing a noble material on a portion of thegas sensing layer between the contacts to act as an ionizing catalystsuch that, upon heating to a temperature, adsorption of a specific gaschanges electronic properties of the gas sensing layer to permitdetection of the gas.
 19. The method as recited in claim 18, wherein theheater includes Yagi Uda antenna elements configured to distribute heatto the gas sensing layer in a uniform manner and further comprisingheating the gas sensing layer with the heater to a temperature betweenabout 150 to about 250 degrees C.
 20. The method as recited in claim 18,wherein sensitivity to a gas to be detected is increased by at least oneof: forming grain boundaries in the gas sensing layer of a particularsize; and selecting a material and geometry for the noble material to besensitive to detection of different gases at different temperatures.